The cotton-top tamarin is a cooperatively breeding monkey living in family groups. Infant care taking is divided among group members with fathers and older siblings doing most infant carrying. While there are significant costs such as weight loss, fathers care for infants from the day of birth. Fathers appear to anticipate infant births with increased prolactin levels prior to birth. Our specific aims are 1.) What is the role of prolactin in nonmaternal infant care and what cues stimulate prolactin? 2.) What is the role of experience in nonmaternal infant care? We will observe natural families to document prolactin's association with helpers during infant care and will manipulate prolactin levels in helpers. We will examine the role of experience with infants as a factor in infant care by studying 12-14 month old tamarins in natural families as they first become involved in infant care, and through tests of experienced fathers housed with infertile females and subsequently tested with reproductive females. We predict that direct exposure to infants will be needed to activate prolactin in young tamarins, but that either infant stimuli or cues from a pregnant female will lead to the response of experienced fathers. We will test responsiveness to infant cues using realistic models of infant tamarins. We will manipulate levels of prolactin to increase levels with hypothesize that elevated levels of testosterone in fathers prior to infant births lead to increased estradiol, which in exogenous prolactin, decrease levels with a dopamine receptor agonist, and test steroid induction of endogenous prolactin. We turn promotes increased prolactin. We will block the aromatization of testosterone to estradiol using an aromatase blocker. We will gather data on the role of oxytocin with non-maternal infant care. Initially, to test the hormonal hypothesis, we will study adult tamarins that have had no prior exposure to infants in their life with and without appropriate hormonal priming. The cooperative infant care seen in cotton-top tamarin families represents the opposite end of a parental care continuum from species where infant care is exclusively maternal. With more than 50 percent of mothers in the US working outside of the home, biparental or cooperative infant care is becoming increasingly necessary. By understanding the interaction of experience and hormones in facilitating care of infants by fathers and non-reproductive helpers in tamarins we hope to provide information of value to human families.